The roll oscillations encountered during the occurrences investigated in this report resulted from a combination of environmental conditions, modified aircraft aerodynamics in the roll axis, and APC. Crosswinds were not considered a factor in these incidents. The weather reports of drizzle at the time of the ACA1130 occurrence suggest water droplet size in the range of 100to 500microns. Based on this, large droplet icing (supercooled large droplet or SLD) was likely encountered by both aircraft on approach to LBPIA, as cloud conditions were similar during each event. If the droplet size was in fact greater than the range of 40to 50microns, the conditions encountered would have been outside the FAR25 AppendixC certification envelope. Although the actual size of SLD at the time cannot be clearly determined, the fact remains that other aircraft types made successful landings under similar conditions. It is, therefore, unlikely that the moderate-to-severe icing conditions alone induced these roll oscillations in the occurrence aircraft. Prior to the occurrences of 07December2002, data regarding the A321aircraft performance in icing were based on its certification and in-service history. As a derivative of the A320,the extent of the A321icing trials was based in part on the A320certification results. Specifically, although the A321has a double-slotted flap system rather than the single-slotted flap arrangement in the A320,both aircraft were considered similar, and the effect of ice accretion on the leading edge of the flaps was not examined during A321certification. Similarly, any A321in-service icing anomalies were viewed in the context of the in-service performance of the entire A320fleet, which revealed no systemic problem. Indeed, for all its certification programmes Airbus considers that ice is accreted in clean configuration. Analysis of the post-occurrence icing trials concluded that ice accretion on the leading edge of the flaps of both the A320and A321, and on the flap tabs of the A321,contributed to unusual roll behaviour by modifying the aerodynamic characteristics. Flights performed in roll direct laws on both aircraft showed that both A320and A321ice accretion with flaps extended induces an increase in roll spoiler efficiency in both CONFIG3 and CONFIGFULL. This increase was higher in CONFIGFULL than in CONFIG3. Further examination of the phenomenon using the normal, lateral flight control laws determined that the increase in roll sensitivity was a significant factor only on the A321aircraft. Specifically, analysis of the normal, lateral flight control laws for both aircraft concluded that the roll sensitivity stability margin on an A321in icing conditions in CONFIG FULL was significantly reduced compared to theA320. Changes to the ELAC software would alter the normal, lateral flight control laws to establish an acceptable roll sensitivity stability margin under these conditions. FAR 25.143(a) and(b)5 require that an aircraft be safely controllable and manoeuvrable, without exceptional piloting skill and without danger of exceeding the aircraft limiting load factor, under any probable operating conditions. The roll sensitivity is a function of pilot input; the normal, lateral flight control law; and the aircraft aerodynamic response. With ACA457 and ACA1130, the ice accretion on the flaps contributed to unusual roll behaviour by modifying the aerodynamic response in the roll axis. This change in aerodynamic characteristics, when combined with normal, lateral flight control law and pilot input, resulted in an unstable aircraft.Analysis The roll oscillations encountered during the occurrences investigated in this report resulted from a combination of environmental conditions, modified aircraft aerodynamics in the roll axis, and APC. Crosswinds were not considered a factor in these incidents. The weather reports of drizzle at the time of the ACA1130 occurrence suggest water droplet size in the range of 100to 500microns. Based on this, large droplet icing (supercooled large droplet or SLD) was likely encountered by both aircraft on approach to LBPIA, as cloud conditions were similar during each event. If the droplet size was in fact greater than the range of 40to 50microns, the conditions encountered would have been outside the FAR25 AppendixC certification envelope. Although the actual size of SLD at the time cannot be clearly determined, the fact remains that other aircraft types made successful landings under similar conditions. It is, therefore, unlikely that the moderate-to-severe icing conditions alone induced these roll oscillations in the occurrence aircraft. Prior to the occurrences of 07December2002, data regarding the A321aircraft performance in icing were based on its certification and in-service history. As a derivative of the A320,the extent of the A321icing trials was based in part on the A320certification results. Specifically, although the A321has a double-slotted flap system rather than the single-slotted flap arrangement in the A320,both aircraft were considered similar, and the effect of ice accretion on the leading edge of the flaps was not examined during A321certification. Similarly, any A321in-service icing anomalies were viewed in the context of the in-service performance of the entire A320fleet, which revealed no systemic problem. Indeed, for all its certification programmes Airbus considers that ice is accreted in clean configuration. Analysis of the post-occurrence icing trials concluded that ice accretion on the leading edge of the flaps of both the A320and A321, and on the flap tabs of the A321,contributed to unusual roll behaviour by modifying the aerodynamic characteristics. Flights performed in roll direct laws on both aircraft showed that both A320and A321ice accretion with flaps extended induces an increase in roll spoiler efficiency in both CONFIG3 and CONFIGFULL. This increase was higher in CONFIGFULL than in CONFIG3. Further examination of the phenomenon using the normal, lateral flight control laws determined that the increase in roll sensitivity was a significant factor only on the A321aircraft. Specifically, analysis of the normal, lateral flight control laws for both aircraft concluded that the roll sensitivity stability margin on an A321in icing conditions in CONFIG FULL was significantly reduced compared to theA320. Changes to the ELAC software would alter the normal, lateral flight control laws to establish an acceptable roll sensitivity stability margin under these conditions. FAR 25.143(a) and(b)5 require that an aircraft be safely controllable and manoeuvrable, without exceptional piloting skill and without danger of exceeding the aircraft limiting load factor, under any probable operating conditions. The roll sensitivity is a function of pilot input; the normal, lateral flight control law; and the aircraft aerodynamic response. With ACA457 and ACA1130, the ice accretion on the flaps contributed to unusual roll behaviour by modifying the aerodynamic response in the roll axis. This change in aerodynamic characteristics, when combined with normal, lateral flight control law and pilot input, resulted in an unstable aircraft. The A321normal, lateral flight control laws programmed into the elevator aileron computer provided higher roll efficiency in CONFIGFULL than in CONFIG3, which resulted in a reduced stability margin in icing conditions. The external influence of the ice on the leading edge of the flaps changed the aircraft aerodynamics, which, when combined with pilot input and the normal, lateral flight control law, resulted in airplane-pilot coupling, which produced an unstable aircraft.Findings as to Causes and Contributing Factors The A321normal, lateral flight control laws programmed into the elevator aileron computer provided higher roll efficiency in CONFIGFULL than in CONFIG3, which resulted in a reduced stability margin in icing conditions. The external influence of the ice on the leading edge of the flaps changed the aircraft aerodynamics, which, when combined with pilot input and the normal, lateral flight control law, resulted in airplane-pilot coupling, which produced an unstable aircraft. Flight tests in natural icing conditions were not accomplished in any configuration in the A321to determine if an acceptable level of safety existed in the handling characteristics. It is likely that the icing conditions encountered by both aircraft were outside the Federal Aviation Regulation25, Appendix C envelopes used for certification of theA321. After these occurrences, flight tests in natural icing confirmed that with flaps extended, ice accretion on the flap leading edges increased roll sensitivity in the normal, lateral flight control law on theA321.Findings as to Risk Flight tests in natural icing conditions were not accomplished in any configuration in the A321to determine if an acceptable level of safety existed in the handling characteristics. It is likely that the icing conditions encountered by both aircraft were outside the Federal Aviation Regulation25, Appendix C envelopes used for certification of theA321. After these occurrences, flight tests in natural icing confirmed that with flaps extended, ice accretion on the flap leading edges increased roll sensitivity in the normal, lateral flight control law on theA321. The flight crews of both aircraft had received up-to-date weather information prior to the approach and landing. The flight crews of both aircraft followed the applicable Air CanadaA321 Aircraft Operating Manual instructions for flight in icing conditions by adding five knots to the VLS. The flight crews were not trained in recovery from airplane-pilot coupling oscillations. Such training was not required by regulation.Other Findings The flight crews of both aircraft had received up-to-date weather information prior to the approach and landing. The flight crews of both aircraft followed the applicable Air CanadaA321 Aircraft Operating Manual instructions for flight in icing conditions by adding five knots to the VLS. The flight crews were not trained in recovery from airplane-pilot coupling oscillations. Such training was not required by regulation. Safety Action Taken Airbus As a result of these occurrences, Airbus issued a Flight Operations Telex (FOT) STL999.138/02 on 20December2002 to all A321operators on the subject of Lateral Control Event During Landing in Icing Conditions. The FOT provided information on the reason for issue, the technical explanation, further action to be taken, an operational recommendation, and follow-up information. Airbus offered the following operational recommendation: When moderate-to-severe icing conditions are anticipated, the use of CONFIG3 for landing is recommended. Time for flight in icing conditions with the flaps extended should be minimized (refer also to [Flight Crew Operating Manual] FCOM3.04.30 page1). In addition, if during the approach, significant ice accumulation is suspected on the non-de-iced parts of the airframe (using the visual ice indicator), the minimum approach speed must not be lower than VLS plus 10knots, and the landing distance should be multiplied by1.15, as already stated in FCOM3.04.30 page1. In June 2003, Airbus issued Operations Engineering Bulletin No.153/1, which provided A321operators with an explanation of potential roll control difficulties being experienced in icing conditions. Essentially, ice accretion on the leading edge of the flaps and flap tabs contributed to unusual roll behaviour by modifying the aerodynamic characteristics in the roll axis. The bulletin went on to repeat its recommendation, that when moderate-to-severe icing conditions are anticipated during approach, CONFIG3 be used for landing. The bulletin also advised that Airbus corrective action consisted of redesign to the elevator aileron computer (ELAC) software (ELACL83 and L91standards) to improve the stability margins of the A321normal, lateral flight control laws. Subsequently, Airbus issued an aircraft flight manual (AFM) revision (AFMTR4.03.00/20) requiring that the operational limitations dealing with using CONFIG3 when landing in icing conditions, previously promulgated through the FOT and Operations Engineering Bulletin No.153/1, be added to the A321AFM. As indicated in its previous communications to operators, between 09March and 04June2004, Airbus issued service bulletins (SBA320-27-1151 and SBA320-27-1152) to provide the required ELAC software updates. Incorporation of these service bulletins will rescind the operational limitations. Air Canada On 09 December 2002, the Chief Pilot, A319/A320/A321 at Air Canada issued A321Aircraft Technical Bulletin No.124. It stated: We are investigating two cases of reduced roll response on approach with significant airframe ice accretion. As a precaution and until further notice, CONFIGFULL landings are prohibited under these conditions. Refer to QRH [Quick Reference Handbook] 2.25for VLS additive with ice accretion. Crews are reminded of the SOP [standard operating procedure] caution: Prolonged flight in icing conditions with slats extended should be avoided. Direction Gnrale de l'Aviation Civile On 15 October 2003, Direction Gnrale de l'Aviation Civile (DGAC) issued Airworthiness Directive2003-388(B), which mandated the A321operational limitations introduced by Airbus' Aircraft Flight Manual TR4.03.00/20. On 18August2004, the DGAC issued Airworthiness DirectiveF-2004-147, which restated the operational limitations imposed by Airworthiness Directive2003-388(B) and further mandated that, before 31December2005, ELACL83 or L91software be implemented in accordance with SBA320-27-1151 or SBA320-27-1152, respectively. Transport Canada and Federal Aviation Administration By 18February2004, both Transport Canada and the Federal Aviation Administration (FAA) had adopted Airworthiness Directive2003-388(B). Transport Canada has adopted DGAC's Airworthiness Directive No.F-2004-147; however, the FAA advises that an FAA Airworthiness Directive will be issued after the FAA's Notice of Proposed Rulemaking process is complete. Safety Concern The redesigned ELAC software should prevent any future A321APC events during similar icing conditions and have a positive effect on future flight control system designs. Despite these technological improvements, the APC phenomenon is not well known within the pilot community. The Board is concerned that the absence of APC recognition within the pilot training syllabus contributes to the lack of awareness of the APC phenomenon.